1. Technical Field
The present invention relates to a method and apparatus for repairing nuclear reactor core shrouds. In particular, the method and apparatus of the present invention are concerned with repair of boiling water reactor shrouds in order to allow safe reactor operation even with extensive cracking along heat affected zones of horizontal welds joining orthogonal shroud plate segments.
2. Discussion of the Prior Art
Boiling water reactor shrouds are disposed concentrically within reactor vessels to divide the flow of coolant water through the reactor vessels and to structurally support and align the fuel assemblies, steam separator assemblies and control rod guide tubes. The shrouds are generally cylindrical and are commonly formed from multiple arcuate steel plates. Plates at each horizontal level of a shroud are joined by welds along abutting, relatively short, vertical edges to form cylindrical segments at respective horizontal levels. Cylindrical segments of the same diameter at adjacent lower levels surround the core region and are joined along their abutting, relatively long, arcuate edges with horizontal welds to form a lower cylindrical section. A horizontal plate shaped like a ring extends radially outward from the top edge of the lower cylindrical section surrounding the core to the bottom edge of an upper cylindrical section of increased diameter to serve as a shoulder or rim for supporting an upper core grid. The horizontal plate can be formed as a single piece or constructed from multiple arcuate pieces welded together to form a ring. Inner and outer circumferential edges of the horizontal plate are joined to the lower and upper cylindrical sections with horizontal welds extending along the entire periphery or circumference of the shroud. An internal flange extends horizontally between the bottom edge of the lower cylindrical section and the top edge of a cylindrical base of diminished diameter to support a core support plate.
After periods of use, cracking of the shroud tends to occur within heat affected zones of the welds as a result of corrosion, radiation and stress. Cracking of the vertically oriented welds is considered acceptable because these welds are relatively short in length, relative to the overall shroud length, and do not adversely affect the function of the shroud (i.e., support and alignment of the nuclear fuel assemblies, and channeling of reactor coolant flow). Specifically, vertical welds at adjacent levels are offset angularly about the shroud periphery so that cracking of such a weld can extend, at most, only the axial or vertical length of that level. However, if cracking occurs along the longer horizontal or circumferential welds, a crack can extend along the entire circumference or periphery of the shroud, permitting relative lateral movement between the plate levels and vertical deflection of the horizontal plate supporting the upper core grid. If, in addition, cracking occurs along radial welds used to join arcuate pieces of the horizontal plate such that the pieces of the horizontal plate can move relative to one another, the structural support needed to position and align the steam separator assembly can be compromised.
More specifically, failure of the welds joining the horizontal plate to respective upper and lower cylindrical sections of the shroud can cause a significant downward deflection of the outer edge of the horizontal plate preventing the upper cylindrical section from adequately supporting the steam separator assembly of the boiling water reactor. In a case where radial welds join pieces of the horizontal plate together, failure of the radial welds joining the horizontal plate pieces can cause even more serious downward deflection of the steam separator assembly. Alternatively or in addition to the downward deflection of the outer edge of the plate, upward deflection of the inner edge of the plate can occur when the lower cylindrical section moves vertically in response to differential pressures across the core support plate as a result of a horizontal crack developing below the core support plate and failure of the horizontal welds joining the horizontal plate to the upper and lower cylindrical sections and/or failure of any radial welds joining horizontal plate pieces. If welds joining segments of the cylindrical base and/or lower cylindrical sections are cracked below the core support plate region, vertical movement of those sections above the crack will promote separation of the cylindrical shroud sections and permit reactor coolant to bypass the core region by flowing through the separations between cylindrical sections of the shroud.
When excessive horizontal and/or radial weld cracking occurs, the shroud must either be replaced or repaired. Repair is certainly the preferred alternative in view of the fact that replacement involves significant expense, relatively long shut-down time, and the potential for radiation exposure to personnel. Repair techniques typically involve bolting brackets onto vertically adjacent plates across a weld crack. This approach requires plural brackets for each crack, depending upon the length of the crack, and is not well suited for repairing cracks and welds joining orthogonal plate segments. Moreover, bolted brackets covering cracks in the weld area of the horizontal plate would require remote cutting and high clamping forces to be held in place.
A superior method for repairing in situ nuclear reactor shrouds of the type having multiple levels of arcuate plates welded together is disclosed in related copending application Ser. No. 08/190,796. The method involves inspecting the shroud for horizontal cracking along welds and securing plural tie-rods in vertical orientation spaced about the periphery of the cracked shroud to axially compress the shroud and thereby urge the opposing surfaces of the horizontal crack toward one another. The repair method and apparatus of the present invention can be used alone or together with the tie-rods of the aforementioned repair method to permit safe reactor operation even with excessive cracking of the horizontal welds joining the horizontal plate to upper and lower cylindrical sections of the shroud. Also, by use of the repair method and apparatus of the present invention, loads and stresses on the horizontal plate are decreased thereby reducing the tendency of the horizontal and/or radial welds of the plate to crack irrespective of whether the method and apparatus are used alone or together with tie-rods.